This invention relates to improved thermoplastic polyester compositions and a method of producing the compositions. The improved thermoplastic compositions comprise an intimate blend of a particulate reinforcing filler with polypivalolactone. The easily processable thermoplastic compositions of the invention have significant property advantages over other reinforced polyester molding resins.
It is well known that certain properties of thermoplastics, including thermoplastic polyesters, may be enhanced by incorporating reinforcing fillers. For example, U.S. Pat. No. 3,814,725 discloses improved polyester molding resins which are obtained by the intimate blending of reinforcing fillers with polypropylene or polybutylene terephthalate. However, to obtain the degree of strength and resistance to deformation at elevated temperatures necessary for the more demanding engineering thermoplastic applications in parts molded from such thermoplastic polyesters, reinforcing fibers of considerable fiber length, usually of the order of one-eighth to one-half inch, must be incorporated in the molding compositions. Such fiber reinforced compositions exhibit very poor surface appearance, which requires in most applications that parts molded from such compositions be painted to obtain a commercially acceptable product. This undesirable characteristic can be overcome through the use of particulate fillers. However, reinforcement with particulate fillers provides only a moderate improvement in strength and deformation resistance at elevated temperatures and often results in a loss of toughness. There is thus a need for thermoplastic polyester compositions having an improved balance of properties, wherein high temperature strength and deformation resistance are combined with good surface appearance and toughness, and wherein acceptable strength and stiffness at ambient temperatures and good processability are retained.